An electrophotographic image forming apparatus that includes a contact charging apparatus may use an electrophotographic photoconductor for forming an electrostatic image thereon and a charging roller that is disposed in contact with the electrophotographic photoconductor.
Here, PTL 1 discloses an image forming apparatus that includes an image carrier for forming an electrostatic image thereon, a charging member that is disposed so as to oppose the image carrier and that charges the image carrier, a pressing unit that applies a pressing force to the charging member towards the image carrier, and gap maintaining members that are mounted on two sides of the charging member, and that contact a surface of the image carrier and maintain a gap between the image carrier and the charging member. Friction force that is produced between the image carrier and each gap maintaining member causes the charging member to be driven and rotated with respect to the image carrier.
In paragraph number 0037, when the outside diameter of the spacer members is Φ12.0 mm, the radius of the cored bar is Φ46.0 mm, and the friction coefficient between the bearing and the cored bar is 0.13, if the axes of the charging roller and the electrophotographic photoconductor drum are horizontal and the rotational speed of the electrophotographic photoconductor drum is 200 rpm, the charging roller rotates if the friction coefficient is greater than 0.065.